355 


GIFT 


LIGHT 

ITSUSE-'MISUSE 


OF   THE 


LIGHT: 
ITS  USE  AND  MISUSE 


A  PRIMER  OF   ILLUMINATION  PREPARED 
UNDER  THE  DIRECTION  OF  THE 
ILLUMINATING  ENGINEER- 
ING SOCIETY 

A* 


COPYRIGHT  1912  BY 

ILLUMINATING  ENGINEERING  SOCIETY 

29  WEST  39th  STREET,  NEW  YORK 


x 


ILLUMINATING 
ENGINEERING  SOCIETY 

THE  ILLUMINATING  ENGINEERING 
SOCIETY  was  organized  in  1906  to  ad- 
vance the  theory  and  practise  of  illuminating 
engineering  and  to  disseminate  knowledge 
relating  thereto.  The  Society  now  has  about 
1600  members  who  are  interested  in  the  sub- 
ject of  lighting  from  various  standpoints  : 
engineering,  economic,  hygienic,  esthetic. 

The  Society  has  no  affiliation  with  any  com- 
mercial organization.  Any  one  interested  in 
its  objects  may  become  a  member. 


LIGHT:  ITS  USE  AND  MISUSE1 


It  is  the  purpose  of  this  publication  to  assist  the  user  in  mak- 
ing artificial  light  effective,  whether  the  light  be  produced  by 
oil,  gas,  electricity  or  otherwise. 

By  proper  use  you  can  get  good  illumination  from  any  of 
these  sources,  but  by  misuse  you  are  likely  to  get  lighting  that 
is  bad,  costly,  and  even  dangerous  to  the  eyesight. 

ILLUMINATION  AND  COMFORTABLE  VISION 

To  see  easily  and  comfortably  you  must  select  the  lamps, 


Fig.  1. — The  eye:  essential  parts  shown  in  section. 

fixtures  and  globes  and  arrange  the  lights  so  as  to  best  suit  the 
particular  conditions  which  have  to  be  met,  but  certain  prin- 
ciples which  must  always  be  followed  may  safely  be  laid  down.2 


Fig.  2.— Pupil  of  eye  ex- 
panded to  let  in  plenty  of  light 
when  illumination  is  dim. 


Same  pupil  contracted  to  shut 
out  excessive  light. 


1 A  primer  of  illumination  prepared  under  the  direction  of  the  Illumi- 
nating Engineering  Society.  (Copyright  1912.)  Applications  for  per- 
mission to  reprint  this  primer  should  be  addressed  to  the  Illuminating 
Engineering  Society,  29  W.  39th  Street,  New  York. 

2  To  understand  these  principles  better,  take  a  glance  at  the  eye  and 


254064 


2  LIGHT  :     ITS    USE    AND    MISUSE 

Don't  Judge  Illumination  by  the  Brightness  of  the  Lamps 

Judge  the  light  you  are  getting  by  the  way  it  helps  you  to 
see.  Do  not  think  because  a  lamp  looks  glaring  and  brilliant 
that  it  is  giving  you  good  light.  It  may  be  merely  giving  you 
too  much  light  in  the  wrong  place.  On  the  other  hand,  a 
well  shaded  table  lamp  may  look  dim  because  it  is  well  shaded, 
and  still  be  giving  first-class  light  for  working  purposes. 

You  must  get  enough  light  to  see  by,  and  as  you  see  things 
chiefly  by  the  light  which  they  reflect,  it  is  evident  that  dark 
colored  objects  which  reflect  light  badly  require  more  light  than 
do  light  colored  objects  to  see  them  comfortably.  That  which 
is  quite  sufficient  for  sewing  on  white  cloth,  for  example,  will 
not  do  at  all  for  working  on  black  cloth. 

Don't  Work  in  a  Flickering  Light 

See  that  your  light  is  steady.  If  you  leave  a  dark  room  and 
go  into  bright  sunshine  the  sensation  is  unpleasant  to  the  eye  ; 
if  you  use  a  light  that  flickers,  you  get  this  same  unpleasant 
sensation,  perhaps  as  rapidly  as  twenty  times  a  minute.  Fur- 
thermore, the  eye  endeavors  to  adjust  itself  to  suit  the  light ; 
if  the  light  flickers  it  keeps  the  iris  of  the  eye  "see-sawing," 
as  it  were,  and  the  muscle  that  governs  it  gets  tired  and  reacts 


see  how  it  works.  Figure  1  shows  the  parts  of  the  eye  as  they  would  ap- 
pear if  it  were  cut  through  from  back  to  front  vertically. 

In  the  process  of  seeing,  the  light  passes  through  the  cornea,  pupil,  and 
lens  of  the  eye  to  the  retina,  just  as  in  a  camera  light  passes  through  the 
lens  to  the  sensitized  film.  The  picture  is  formed  on  the  retina,  which  is 
a  layer  made  up  of  the  ends  of  nerve  fibers  which  gather  into  the  optic 
nerve  and  go  directly  to  the  brain.  The  optic  nerve  sends  along  the  pic- 
ture to  the  brain  for  notice.  The  lens  of  the  eye,  unlike  that  of  the  camera, 
automatically  changes  in  thickness  to  focus  or  make  a  clear  image  on  the 
retina  for  seeing  at  different  distances.  This  focusing  action  is  called  the 
accommodation  of  the  eye,  and  when  the  light  is  dim  or  bad  the  focusing 
muscle  vainly  hunts  for  some  focus  which  may  make  objects  look  clear  and 
gets  tired  in  trying  to  do  it.  The  muscles  which  move  the  eye  about  also 
get  tired  in  the  same  way  and  the  result  is  eye-strain,  which  stirs  up  pain 
and  headache  just  as  any  other  over-tired  muscles  of  the  body  may  set 
up  an  ache. 

The  iris  (which  gives  the  eye  its  color)  serves  to  regulate  the  amount 
of  light  which  reaches  the  eye.  In  very  dim  light  it  opens  out  making  the 
pupil  big,  as  shown  in  figure  2,  and  in  very  bright  light  it  shuts  up  as 
shown,  and  thus  keeps  out  a  flood  of  brilliant  light  which  might  hurt  the 
retina.  The  protective  action  of  the  pupil  is  pretty  good,  but  by  no  means 
complete,  for  it  seldom  gets  smaller  than  shown  in  the  illustration,  how- 
ever bright  the  light. 


LIGHT:    ITS  USE  AND  MISUSE  3 

on  the  nerves  to  cause  discomfort  and  pain.  Reading  in  railway 
trains  causes  similar  strain ;  the  eye  muscles  get  tired  in  trying 
to  follow  the  shaking  page,  and  are  likely  to  provoke  a  headache. 

Don't  Expose  the  Eyes  to  an  Unshaded  Light 
It  is  bad  to  have  an  unshaded  brilliant  light  glaring  into  the 
eyes,  for  it  throws  hard  labor  upon  them  in  an  effort  at  adjust- 
ment.    This  applies  even  to  common  electric,  gas,  or  oil  lamps. 
(See  figs.  3,  4,  5.)     While  artificial  light  may  be  made  a  good 


Fig.  3.— Very  bad  lighting.    This  man  receives,  full  in  the  face,  both 

direct  light  from  the  unshaded  lamp  and  reflected 

glare  from  the  table  top  and  papers. 

substitute  for  daylight,  you  have  constantly  to  beware  lest  rays 
that  are  too  bright,  either  from  the  lamps  or  from  their  reflec- 
tions, hurt  the  eyes.  You  can  get  reflections,  so  bright  as  to 
be  harmful,  from  polished  metal  or  glass,  from  bright  varnished 
surfaces,  or  even  from  glossy  white  paper  upon  which  the 
light  falls. 

A  bright  light  fairly  in  the  field  of  view  means  a  very  brilliant 
light  on  the  retina,  producing  fatigue.  Everyone  knows  the 
blinding  sensation  of  looking  at  the  sun  with  its  sequence  of 


LIGHT:    ITS  USE  AND  MISUSE 


dazzling  colored  images.  Babies  are  here  common  sufferers 
when  careless  mothers  or  nurses  allow  them  to  lie  in  their  car- 
riages with  eyes  exposed  to  the  unclouded  sun.  Bright  arti- 
ficial lights,  in  a  less  degree,  do  the  same  thing  to  all  of  us. 
And  when  you  get  a  bright  light  in  the  field  of  view,  the  pupil 
tries  to  shut  it  out ;  in  so  doing  it  renders  less  bright  things 


Fig.  4. — Faulty  arrangement  of  dining-room  lighting.    The  lamps  exposed 

in  the  dome  shine  in  the  eyes.     Trouble  is  aggravated  by 

general   darkness   of  the  room. 

all  but  invisible.  Thus  it  is  hard  to  see  things  which  are 
nearly  in  line  with  a  brilliant  light,  as  you  often  find  in  facing  an 
automobile  headlight,  or  looking  into  a  show  window  like  fig.  6. 

Place  an  unshaded  lamp  in  front  of  a  picture 
A  Couple  of  on  the  wall  and  then  gtand  back  a  f ew  f eet  and 

Simple  Experiments 

note  how  much  of  the  picture  you  can   see 

clearly.  Then  hold  a  book  or  paper  at  arm's  length  so  as  just 
to  cover  the  bright  light  and  note  the  way  in  which  the  pic- 


LIGHT:   ITS  USE  AND  MISUSE  5 

ture  clears  up.  Again,  put  an  unshaded  lamp  about  a  foot  in 
front  of  your  eyes  and  try  to  read  a  newspaper  just  beyond  it. 
Then  shade  your  eyes  from  the  lamp  and  try  it  again.  You 
will  soon  find  out  in  this  way  that  lamps  can  be  so  placed  that 
they  will  be  a  hindrance  rather  than  a  help  in  seeing.  (Com- 
pare fig.  7  with  fig.  6.) 

Prom  time  immemorial  mankind  has  received 
its  light  mostly  from  the  sky.     Consequently 
the  part  of  the  retina  on  which  the  light  from 
above  chiefly  falls  is  pretty  well  used  to  it,  while  bright  light 


Best  Direction 
of  Light 


Fig.  5.— These  ladies  are  annoyed  by  the  glare  of  the  unshaded  light  when 

they  look  across  the  room.    Common  and  faulty  arrangement. 

Lamps  should  be  enclosed  in  diffusing  globes. 

from  below,  falling  on  the  part  of  the  retina  which  commonly 
gets  light  only  from  grass  or  dark  pavements;  may  be  very  ir- 
ritating and  unpleasant.  Thus  the  glare  from  snow  and  sand 
is  not  only  disagreeable  on  account  of  its  intensity  but  because 
of  the  unusual  direction  from  which  it  comes.  Just  so  with  a 
brilliant  beam  reflected  from  glossy  paper  on  which  you  are 
writing.  Its  rays  strike  you  from  an  unusual  direction  and 
are  harmful  for  that  reason.  Other  smooth  and  shiny  sur- 


LIGHT:    ITS  USE  AND  MISUSE 


Fig.  6. — Poor  arrangement  for  display.    You  see  the  lamps 
instead   of    the    sweets. 


Fig.  7. — Excellent  arrangement  for  display.    No  lamps  in  sight. 
Every  garment  is  brightly  lighted. 


LIGHT:    ITS  USE  AND  MISUSE  7 

faces  deliver  an  equally  hurtful  assault  on  that  sensitive  and 
much  abused  organ,— the  eye. 

Don't  Read  Facing  the  Light 

It  is  best  to  have  the  light  come  from  above  and  somewhat 
sidewise,  as  it  commonly  does  in  nature,  so  that  you  will  not 
get  a  brilliant  reflection  or  glare  from  what  you  are  trying  to 
see.  In  reading  and  writing  it  is  better  to  have  the  light  come 
from  the  left,  to  avoid  getting  a  shadow  of  the  hand  that  holds 
the  book  or  pen.  Let  the  lamp  be  just  far  enough  behind  to 
keep  direct  reflections  from  the  paper  out  of  the  eyes.  (See 
figs.  8,  9,  14,  15.)  But  what  has  been  said  of  reflections  from 
paper  applies  with  even  more  force  to  the  case  of  polished  metals, 
or  the  like,  over  which  one  is  busy.  Individual  lights  placed  close 
over  the  work  are  very  likely  to  produce  these  troublesome 
direct  reflections  and  consequently  such  lights  are  falling  into 
disuse.  In  an  interior  otherwise  dark,  their  use  is  open  to  the 
further  objection  of  giving  bright  spots  of  light  and  so  pro- 
ducing too  violent  contrasts  of  light  and  shade.  (See  figs.  16, 17.) 

Don't  Use  a  Bright  Light  Against  a  Dark  Background 
Almost  any  light  will  glare  unpleasantly  if  the  surroundings 
are  thoroughly  dark.  As  an  extreme  illustration,  the  light 
from  a  big  arc  lamp  hung  close  to  the  sidewalk  may  be  very 
annoying  at  night,  but  by  day  you  would  hardly  notice  it.  Just 
so  a  bright  lamp  against  a  dark  background  may  be  annoying, 
while  against  a  light  background  it  would  not  be  so  unpleasant. 

LAMPS,  FIXTURES,  GLOBES,  REFLECTORS 

One  may  choose  to-day  among  lights  of  many  kinds.  There 
are  at  hand  candles,  oil  lamps,  open  flame  gas  jets,  upright  and 
inverted  mantle  gas  lamps,  electric  incandescent  lamps  of  car- 
bon and  of  tungsten,  electric  arcs  of  half  a  dozen  varieties, 
besides  mercury- vapor  tubes,  acetylene  lamps,  as  used  on  mo- 
tor-cars, and  so  on  through  a  long  list. 

What  do  you  wish  to  use  a  light  for  ?  To  read  or  write  by, 
to  bring  into  view  the  working  parts  of  a  machine,  to  match 
colors,  to  display  goods,  or  pictures ;  or  merely  to  make  a  path- 
way safe  and  plain  ?  Each  case  is  to  be  studied  by  itself,  and 
the  effect  is  to  be  accomplished  by  such  lamps,  globes  and  re- 
flectors as,  properly  disposed,  will  insure  ample  lighting  with- 
out glare,  and  yet  with  strict  economy. 


8 


LIGHT:   ITS  USE  AND  MISUSE 


However  good  and  suitable  the  lamp,  it  will  be  put  at  a  dis- 
advantage unless  the  lighting  fixture  which  carries  it  is 
designed  to  hold  the  lamp  in  the  right  position  to  enable  one 
to  best  utilize  the  light  which  comes  from  it.  Prettiness  in  a 
fixture  is  well  enough  ;  but  let  the  fixture  be  serviceable  first ; 
then  it  may  be  also  as  pretty  as  you  please.  But  don't  buy 
prettiness  if  it  makes  war  on  good  service. 

Daylight  is  naturally  well  diffused  ;  but  artificial  light,  poured 


Fig.  8.— A  bad  position  for  reading.    In  spite  of  the  shaded  lamp,  glare 
from  paper  reflected  into  eyes,  is  very  trying  and  harmful. 

out  as  it  is  from  mere  points,  or  narrow  surfaces,  needs  to  be 
tempered  or  softened  by  shades.  And  it  sometimes  further 
requires  to  be  directed  upon  a  desk  or  table  or  other  object. 
In  some  cases  it  is  better  to  adopt  indirect  methods,  and  throw 
the  beams  of  a  lamp  upon  a  ceiling  whence  the  rays  are  scat- 
tered. For  every  particular  need  there  is  ample  provision 
amid  the  abounding  lamps,  globes  and  reflectors  of  present 
day  designers. 


Arranging  Lights 


LIGHT:   ITS  USE  AND  MISUSE  9 

Two  methods  are  usual  in  arranging  lamps  : 
first,  to  secure  general  illumination  by  so  plac- 
ing the  lamps  that  you  may  see  with  comfort  anywhere  in  a 
room  ;  second,  in  cases  where  a  bright  light  is  not  necessary 
throughout  a  room,  local  illumination  can  be  planned,  placing 
the  lights  where  they  will  be  most  used,  always  remembering 


Fig.  9. — Good  position  in  reading.    No  light  directly  hits  the  eyes 
and  no  glare  is  reflected  from  the  book. 

that  it  will  not  do  to  localize  light  too  much,  since  you  need 
for  comfortable  seeing  a  fair  quantity  of  light  broadly  dis- 
tributed. 


PRECAUTIONS  TO  BE  TAKEN  IN  PLACING 
AND  SHADING  LIGHTS 

In  any  artificial  lighting  the  lamps  should  be  so  well  shaded 
that  the  eye  does  not  see  them  directly  nor  brilliant  reflections 


io  LIGHT:    ITS  USE  AND  MISUSE 

from  them.  You  can  accomplish  this  end  by  putting  the  light 
Diffusing  and  in  diffusing  globes  of,  for  instance,  ground  glass 

Shading  by  Globes  or  white  or  opal  glass  or  other  translucent  ma- 
and  Reflectors  terial.  To  secure  the  best  diffusion,  the  globes 

should  be  dense  enough  not  to  reveal  the  form  of  the  actual 
light  source  within,  but  to  give  the  effect  of  the  light  pouring 
forth  from  the  globe  as  a  whole.  (See  fig.  10.) 

Another  way  of  accomplishing  the  same  result  is  to  put  a 
shade  around  the  lamp,  which  screens  it  and  reflects  downward 
much  of  the  light  which  would  otherwise  idly  fall  on  the  walls 
or  ceiling.  (See  fig.  11.)  Such  shades  may  be  of  mirrors  or 


Opal  glass  globe.  Ground  glass  globe. 

Fig.  10. — Within  each  globe  is  a  lamp  of  the  same  candle-power. 
Note  the  superior  diffusion  of  the  light  by  the  opal  globe. 


polished  metal  or  white  or  opal  glass,  of  ground  or  prismatic 
glass,— all  of  which,  in  a  measure,  work  alike.  Glass  shades 
are  generally  preferable  to  metal  ones,  for  a  little  light  pene- 
trates them  sidewise,— just  enough  to  keep  the  upper  part  of 
the  room  from  being  too  dark. 

Another  scheme  successfully  used  to  shield  the 
light  is  to  turn  the  light  from  the  lamp  upward 
on  to  the  ceiling  by  means  of  an  opaque  reflector  underneath. 
The  reflector  conceals  the  lamp,  and  the  brightly  illuminated 
ceiling  by  which  the  light  is  diffused  serves  as  the  actual 
source  of  the  illumination.  (See  fig.  12.)  This  plan  should 
be  used  only  on  white  or  very  light  ceilings  and  is  subject  to  a 
heavier  loss  for  securing  diffusion  than  some  other  methods, 


LIGHT:    ITS  USE  AND  MISUSE  u 

but  often  this  loss  is  reimbursed  by  the  thoroughness  with 
which  the  ceiling  diffuses  the  rays  that  fall  upon  it. 

Don't  Use  Local  Lighting  by  Itself 

In  ordinary  cases  general  illumination  is  the  best  way  of 
lighting  an  interior  unless  some  of  the  work  in 

General  Illumination   han(^  ag  sewing  dark  goods  or  reading  very  fine 

print,  demands  exceptionally  strong  lighting  in 


Fig.  11.— General  illumination  by  direct  lighting;  lamps  concealed 

in  diffusing  glass  reflectors. 

. 

some  parts  of  the  room.  In  this  case  local  lights  may  be  added, 
but  they  .ought  not  to  be  used  without  pretty  strong  general 
illumination.  The  commonest  sort  of  localized  lighting  is  that 
furnished  by  a  table  lamp.  Such  a  lamp  should  always  be 
shaded  to  keep  the  direct  light  out  of  the  eyes,— best  by  a 
translucent  shade  which  will  add  something  to  the  general 
illumination. 

In  any  one  of  these  plans  for  general  illumination,  lights 
should  be  so  placed  as  to  give  at  least  fairly  uniform  lighting 


12 


LIGHT:    ITS  USE  AND  MISUSE 


everywhere  in  a  room,  otherwise  there  may  be  strong  and  jar- 
ring contrasts  of  light  and  darkness. 

In  using  shades  open  at  the  bottom,  such  as  are  very  com- 
mon, their  shape  and  character  can  be  so  chosen  as  to  distribute 
the  light  precisely  as  desired ;  this  result  can  in  less  degree 
be  accomplished  by  using  enclosed  globes  or  by  indirect  lighting. 


Fig.  12.— General  illumination  by  indirect  lighting;  lamps  are  concealed 
in  opaque  reflectors  and  the  light  is  diffused  from  the  ceiling. 


Any  of  the  schemes  here  sketched  can  be  made  to  give  good 
results.  The  choice  between  them  turns  upon  just  what  task 
is  required  of  the  light  and  what  its  surroundings  are  to  be. 
Ordinarily,  lighting  from  shades  open  at  the  bottom  gives  a 
stronger  light  than  other  methods,  but  you  must  carefully 
avoid  glare  in  these  cases.  Lighting  by  wholly  indirect  means, 
in  which  all  the  rays  are  diffused  from  the  ceiling,  demands 
lamps  of  extra  power  for  the  same  illumination,  but  requires 
little  care  to  avoid  glare.  Rooms  lighted  from  diffusing  globes 
take  an  intermediate  position  with  respect  to  freedom  from  glare. 


LIGHT:   ITS  USE  AND  MISUSE 


(a)  Concentrating  re- 
flector :  lights  a  small 
area  brilliantly. 


/b)  Semi-concentrat- 
ing reflector:  lights  a 
larger  area  less  bril- 
liantly. 


(c)  Distributing  re- 
flector: lights  a  wide  area 
moderately. 


— — — — —  •--  -•  —  ^—•^—^•^•1 

Fig.  13.— Reflectors  put  the  light  where  you  want  it;  (a),  (b),  and  (c) 
have  lamps  of  the  same  candle-power.  (These  pictures  are  in- 
tended only  to  show, in  a  general  way,  the  effect  of  different  types 
of  reflectors.) 


14  LIGHT:    ITS  USE  AND  MISUSE 

For  a  lamp  to  do  its  best  work,  it  should  not  be 

Misplaced  Brackets  .   „       .„  ;,  . 

too  near  a  wall,  especially  if  this  wall  is  dark. 
Hence  only  very  small  rooms  can  be  well  lighted  by  the  usual 
side  brackets,  say  4  to  6  feet  high.  In  a  large  room  the  eye 
cannot  avoid  glare  from  such  brackets  unless  their  lamps  are 
so  heavily  shaded  as  to  dim  the  room.  In  large  rooms  where 
brackets  can  be  placed  high  enough  to  be  out  of  the  general 


Fig.  14. — Don't  place  a  desk  lamp  like  this;  it  glares  from  the 
paper  and  shines  in  your  eyes  besides. 

view,  they  may  be  used  to  advantage ;  and  they  are  often 
convenient  for  occasional  use,  as  in  bedrooms,  when  the  room 
is  also  lighted  by  other  means. 

ECONOMY  AND  EFFICIENCY 
Don't  Waste  Light  by  Using  the  Wrong  Reflectors 
By  using  reflectors  you  can  put  the  light  from  a  lamp  where 
it  will  do  the  most  good,  much  as  an  automobile  headlight 


LIGHT:    ITS  USE  AND  MISUSE  15 

sends  the  light  along  the  road  just  where  it  is  wanted.  In 
fig.  3  there  is  a  lamp  without  any  shade  or  reflector  and 
you  will  see  that  the  light  goes  in  all  directions,  only  a 
small  part  of  it  falling  on  the  level  of  the  table  where  it  is 
needed.  The  rest  hits  the  walls  and  is  reflected  about  the 
room  losing  intensity  at  each  reflection.  Obviously  an  un- 
shaded lamp  does  not  throw  the  light  where  it  is  wanted.  To 
ensure  the  light  falling  upon  the  table  you  must  use  a  reflec- 
tor that  will  bring  it  there. 


Fig.  15.  —  If  you  must  use  a  desk  lamp,  put  it  in  this  position.     If  an  open 

reflector  is  used  let  it  be  of  the  diffusing  type.    Better  still  is  a 

reflector  with  a  diffusing  glass  bottom. 


Of  such  reflectors  there  are  three  general  types,  either  of 
glass  or  of  metal,  which  we  may  call  concentrating,  semi-con- 
centrating, and  distributing.  (See  fig.  13,  (a),  (b)  and  (c).) 
The  first  acts  almost  like  an  automobile  headlight,  throwing 
its  light  downward  into  a  comparatively  small  area.  The  sec- 
ond kind  spreads  out  the  light  over  a  much  wider  area,  of 
diameter  perhaps  as  great  as  the  height  of  the  lamp  above  the 


i6 


LIGHT:    ITS  USE  AND  MISUSE 


table,  while  the  third  is  planned  to  light  a  comparatively  big 
area  not  very  intensely  at  any  one  spot. 

No  reflector  ever  increases  the  total  light  that  streams  out 
of  a  lamp  ;  it  only  puts  the  light  where  it  is  needed  instead  of 
letting  it  go  unguided. 


Fig.  16.— More  light  in  the  eyes  than  on  the  work  and 
not  enough  light  in  the  room.  Sharp  shadows  and 
much  glare  from  the  polished  metal.  Discomfort 
to  the  worker:  loss  to  his  employer. 


Don't  Use  Shallow  Reflectors 

All  reflectors  should  come  far  enough  down  over  their  lamps 
to  prevent  you  from  seeing  the  bright  sources  of  light  them- 
selves without  actually  looking  upward. 

Hekht  of  Lam  *        With  proper  reflectors,  their  height  above  the 
table,  counter,  or  bench  ordinarily  makes  little 


LIGHT:    ITS  USE  AND  MISUSE  17 

difference  since  it  is  the  purpose  of  the  reflectors  to  send  the 
light  where  it  will  do  the  most  good. 

Because  dark  walls  absorb  light  strongly  in- 
stead of  reflecting  it  they  demand  much 
stronger  lamps  for  sufficient  illumination  than 
do  light  walls.  (See  fig.  18.)  A  very  dark  wall-paper  or  a 
dark  wood  finish  may  require  three  or  four  times  as  much 
light  as  a  really  light  finish.  Dark  reds,  greens,  and  browns 
reflect  pnly  10  to  15  per  cent,  of  the  light  which  falls  on  them. 


Effect  of  Dark  Walls 
and  Colored  Globes 


Fig.  17.— Example  of  good  general  illumination  in  a  factory;  the  whole 
area  is  uniformly  and  brightly  lighted. 

White,  cream  color,  and  light  yellowish  tints  may  reflect  over 
one-half  the  light. 

Likewise,  deeply  tinted  globes  and  shades  absorb  much  light,— 
a  fact  which  must  be  borne  in  mind  in  considering  economy. 

Don't  Let  Lamps  and  Globes  Get  Dirty 
Dirt  on  lamp  chimneys,  electric  bulbs,  globes,  or  reflectors 
absorbs  and  wastes  much  light.    The  country  over,  it  is  safe  to 


1 8  LIGHT:    ITS  USE  AND  MISUSE 

say  that  millions  of  dollars  are  wasted  every  year  by  letting 
lamps  become  foul  and  dust  laden.  Nor  is  there  any  economy 
in  using  electric  bulbs  until  they  blacken.  It  pays  to  renew 
promptly  blackened  bulbs  and  defective  gas  mantles. 

Don't  Save  Light  at  the  Expense  of  Your  Eyes 

Saving  light  at  the  cost  of  eyesight  is  false  economy.     To 

Real  and  False          get  good  lighting  it  is  generally  necessary  to 

Economy  diffuse  the  light  from  the  lamps  either  directly, 

by  opal  or  ground  glass  shades,  or  indirectly,  by  turning  the  light 


Fig.  18. — Each  of  these  two  little  rooms  receives  the  same 

light.     Dark  walls  absorb  most  of  the  rays  of 

light  in  left-hand  room. 

first  on  the  ceiling  or  wall.  The  use  of  ground  glass  involves  the 
absorption  of  15  to  20  per  cent,  of  the  light  to  secure  diffusion, 
opal  glass  of  various  kinds  from  20  to  40  per  cent.,  while  some 
forms  of  art  glass  and  most  diffusing  ceilings  absorb  more  than 
half  the  light  that  falls  upon  them.  Even  though  all  these  ap- 
pliances absorb  light  in  the  process  of  diffusion,  there  is  gain  in 
their  use  because  they  yield  rays  more  grateful  to  the  eyes. 
But  your  eyes  may  tire  easily  even  with  good  lighting.  If  so, 


LIGHT:    ITS  USE  AND  MISUSE  *9 

consult  an  oculist  and  don  glasses  if  you  need  them.    Eye-strain 
often  comes  from  defective  eyes  as  well  as  from  faulty  lights. 

In  gas  lighting  there  is  no  economy  in  using 
Economy  in  . 

Selecting  Lamps         open  name  burners  unless  the  exposure  is  such 

that  gas  mantles  would  often  be  broken.  Sim- 
ilarly, with  electric  lights  it  is  very  wasteful  in  most  cases  to 
use  carbon  filament  lamps  when  tungsten  lamps  are  available. 
The  carbon  lamps  cost  less  to  install  or  replace,  but  much 

more  for  electric  current.1 

• 

AMOUNT  OF  ILLUMINATION  REQUIRED 

The  common  unit  of  illumination  is  the  foot-candle,  mean- 
ing thereby  the  light  which  the  object  would  receive  from  a 
standard  candle  at  the  distance  of  one  foot.  This  is  the  meas- 
uring rod,  as  it  were,  by  which  comparisons  are  made. 

No  absolute  rule  can  be  laid  down  for  the  number  of  foot- 
candles  required  for  good  seeing.  Individuals  differ  widely  in 
their  requirements;  and  the  conditions  under  which  the  light 
is  used  cause  still  greater  variations  in  requirement.  How- 
ever, where  lighting  arrangements  are  well  planned  it  has  been 
found  by  experience  that  ordinary  reading,  writing,  or  work  on 
white  or  light  colored  material,  can  comfortably  be  carried  on 
by  most  people  with  an  illumination  of  2  to  3  foot-candles.  For 
sewing  dark  goods,  or  reading  fine  type,  5  foot-candles  are  none 
too  much,  while  for  drafting,  engraving,  watchmaking,  work- 
ing on  black  cloth,  and  the  like,  from  7  to  10  foot-candles  should 
be  furnished. 

1  The  amount  of  electricity  taken  by  an  electric  lamp  is  expressed  in 
watts.  Most  electric  lamps  now  manufactured  have  the 
number  of  watts  which  they  are  rated  to  consume 
printed  on  a  label  on  the  bulb.  The  old-fashioned  car- 
bon filament  incandescent  lamp  of  16  candle-power  has  the  candle-power 
on  the  label,  and  takes  from  50  to  60  watts. 

To  determine  the  cost  of  operating  an  electric  lamp,  divide  the  number 
of  watts  it  consumes  by  1000  to  reduce  to  kilowatts,  and  multiply  the 
number  of  hours  the  lamp  is  to  be  operated  by  the  kilowatts  to  obtain 
the  kilowatt-hours  of  electrical  energy.  The  kilowatt-hours  multiplied  by 
the  rate  per  kilowatt-hour  which  is  charged  gives  the  cost  of  operation 
for  the  stated  time. 

The  consumption  of  gas  lamps  is  expressed  in  cubic  feet  of  gas  per  hour. 
The  number  of  cubic  feet  of  gas  per  hour  taken  by  a  burner,  divided  by 
1000,  and  multiplied  by  the  cost  per  thousand  cubic  feet  of  gas,  and  by 
the  hours  of  burning,  gives  its  cost  of  operation  for  the  stated  time. 

The  consumption  of  open  flame  burners  is  commonly  taken  at  5  cubic 
feet  per  hour.  Upright  single  mantle  burners  usually  take  from  3^  to  5 
cubic  feet  per  hour,  though  some  smaller  ones  take  less.  Most  individual 
inverted  gas  mantle  burners  take  from  3  to  3^  cubic  feet  per  hour. 


26 


'     LIGHTS  'ITS    USE    AND    MISUSE 


Artistic  Effects 


In  a  room  suitably  arranged  for  comfortable 
seeing,  you  may  have  plenty  of  light,  but  the 
general  effect  may  be  displeasing.  The  illumination  may  quite 
fail  to  bring  out  the  good  points  of  the  room  in  architecture 
and  decoration,  or  may  play  pranks  with  the  appearance  of 
persons  or  things  in  the  room.  (See  fig.  19.)  One  may  not 
object  to  ghastly  tints  in  a  factory,  but  in  lighting  a  drawing 
room  such  effects  would  not  be  tolerated.  Hence  one  often 


Bust  lighted  from  above  and  in 
front. 


The  same  bust  lighted  from  di- 
rectly overhead. 


Fig.  19.— Bad  lighting  defeats  good  art. 


should  sacrifice  strict  economy  to  get  the  most  pleasing  effect 
in  the  room.  The  fixtures  that  carry  the  lights  should  har- 
monize with  their  surroundings  if  the  general  effect  is  to  be 
agreeable.  Handsome  fixtures  have  a  decided  decorative  value 
whether  their  lamps  are  lighted  or  not.  As  strongly  colored 
objects  give  something  of  their  own  hue  to  all  the  light  which 
they  reflect,  the  color  of  lamp  shades,  walls,  and  furnishings 
plays  an  important  part  in  the  artistic  effect. 


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